1. Technical Field
The technical field relates to a proximity contactless communication apparatus for use in baseband proximity contactless communication through electromagnetic coupling between respective antennas of two communication apparatuses disposed close to each other. The present disclosure also relates to a proximity contactless communication system including such proximity contactless communication apparatuses, and relates to a proximity contactless communication method using such proximity contactless communication apparatuses.
2. Description of Related Art
In recent years, the transmission rate of high-speed digital interfaces has been increasing steadily. There are interfaces to be connected through a cable, such as USB 3.0 interface having a transmission rate of 5 Gbps, and S-ATA 3.0 (Serial ATA 3.0) interface having a transmission rate of 6 bps. There are interfaces for removable memory cards, such as UHS-II (Ultra High Speed Phase-II) interface for SD memory cards, having a transmission rate of 1.56 Gbps.
FIG. 22 is a schematic diagram showing a memory card 102 and a host apparatus 101 according to prior art. FIG. 23 is a diagram showing that the memory card 102 of FIG. 22 is inserted into a socket of the host apparatus 101. FIG. 24 is a cross-sectional view along a line A3-A4 of FIG. 23. The host apparatus 101 is provided with a circuit board 111, a communication circuit 112, transmission lines 113, and electrodes 114, as shown in FIG. 24. The memory card 102 has terminals P1 to P9 exposed on its surface, as shown in FIG. 22. In addition, the memory card 102 is provided with a circuit board 121, a communication circuit 122, a flash memory 123, transmission lines 124, and the electrodes P1 to P9, as shown in FIG. 24. Although FIG. 24 only shows the electrode P1 of the memory card 102 and the electrode 114 of the host apparatus 101 for ease of illustration, the other electrodes P2 to P9 of the memory card 102 and corresponding electrodes of the host apparatus 101 are also provided in a similar manner. When the memory card 102 is inserted into the host apparatus 101, the electrode P1 of the memory card 102 electrically contacts the electrode 114 of the host apparatus 101, and the communication circuit 112 of the host apparatus 101 is connected to the communication circuit 122 of the memory card 102, through the transmission line 113, the electrode 114, the electrode P1, and the transmission line 124.
The higher the transmission rate of an interface increases, the more significantly respective portions included in a signal transmission path affect signal quality. For example, electrical contacts included in the signal transmission path cause degradation in signal quality. In case of the memory card 102 and the host apparatus 101 of FIGS. 22 to 24, in particular, in a transmission path between the communication circuit 112 of the host apparatus 101 and the communication circuit 122 of the memory card 102, it is more difficult to design the characteristic impedances of the electrodes 114 and P1, than to design the characteristic impedances of the transmission lines 113 and 124. It results in a mismatch of characteristic impedance at a contact between the electrode P1 of the memory card 102 and the electrode 114 of the host apparatus 101, thus degrading signal quality.
In addition, since the electrodes P1 to P9 of the memory card 102 may contact with the human body, an I/O circuit (not shown) of the communication circuit 122 is connected to the electrodes P1 to P9 through electrostatic protection elements (not shown). Since each electrostatic protection element generally has a capacitive component of several pF, the characteristic impedance of this portion significantly decreases as compared to the characteristic impedances of other portions included in the signal transmission path, thus degrading signal quality.
Hence, in order to further increase the transmission rate of the interface for removable memory cards, as an alternative, it is proposed to use a high-speed digital interface using proximity contactless communication.
There are proximity contactless communication methods, such as: a method of transmitting a radio-frequency carrier wave modulated by a baseband digital data signal, such as Wi-Fi (wireless fidelity), (radio frequency proximity contactless communication); and a method of just transmitting a baseband digital data signal through electromagnetic coupling between two antennas disposed close to each other (baseband proximity contactless communication).
If using proximity contactless communication in order to increase the transmission rate of the interface for removable memory cards, it is desirable to use baseband proximity contactless communication. A carrier wave base clock source and a modulator circuit are required for radio frequency proximity contactless communication. On the other hand, a carrier wave base clock source and a modulator circuit are not required for baseband proximity contactless communication, and thus, there is a great advantage in terms of cost. Baseband proximity contactless communication can be implemented by connecting antennas to a communication circuit of a memory card and to a communication circuit of a host apparatus according to prior art, respectively, disposing the two antennas close to each other, and just transmitting a baseband digital data signal through electromagnetic coupling between the antennas. There is a communication system capable of baseband proximity contactless communication, such as an invention of Patent Literature 1.